Biaxially oriented polyester films in the range of thickness from 3 to 200 μm are well known in the form of transparent, matt, and white films. These films are produced with pigments, e.g. silicon dioxide, calcium carbonate, barium sulfate, kaolin, titanium dioxide, aluminum oxide, or combinations thereof. These films are generally not thermoformable, do not exhibit diffuse scattering power for visible light, and do not reflect IR radiation.
EP 0 659 198 B1 describes composite materials intended for solar radiation purposes, screening purposes, and, respectively, filtering purposes, uses described being agriculture and horticulture. These films are comprised of a transparent polymer selected from the group of low- or high-density polyethylene, ethylene-vinyl acetate copolymer, polytetrafluoroethylene, polyvinylidene dichloride, polyvinyl chloride, polycarbonate, polymethyl methacrylate, and mixtures thereof. These films comprise from 0.1 to 30% by weight of an interference pigment which is comprised of a lamellar material, which has been coated with one or more metal oxides. These films feature selective screening of radiation, which has a favorable effect on plant growth.
EP 0 791 620 A2 describes films for agricultural applications comprised of polyetheramide block copolymers, of thermoplastic polyester elastomers, or of thermoplastic polyurethanes, having water-vapor permeability of from 100 to 25 000 g/m2/24 h, transmittance of from 20 to 90% for visible light, and transmittance of from 5 to 90% for thermal radiation with wavelength of 5 μm. The films comprise lamellar inorganic particles which have been coated with a substance with a high refractive index.
The films described have not been biaxially oriented and have inadequate longitudinal and transverse mechanical properties, making them unusable for many applications where the demands are high ultimate tensile strength, high modulus of elasticity, high tensile strain at break values, and high tensile stress values to generate 5% tensile strain (F5 value).
The films described are moreover unsuitable for the thermoforming process, because the result would be uncontrolled orientation of the pigments partially orientated via melt flow. This would have an uncontrolled effect on optical properties of the film, e.g. gloss, haze, and mattness, via vacuole formation involving pigments and orientation of the film.
These films moreover yellow as a result of prolonged insolation, making them unsightly. The mechanical properties of the films are also impaired by insolation, and the films therefore become brittle after a very short time. This effect, brought about by the short-wave fraction of sunlight, is very much more pronounced in pigmented films, in particular in films which comprise photoactive metal oxides, e.g. titanium dioxide, than in unpigmented films.
EP 1 251 369 A2 describes biaxially oriented multilayer polyester films whose base layer comprises an IR-absorbent dye, and which are suitable for use as IR filters. This dye has an absorption maximum at from 800 to 1000 nm, and gives the film maximum transmittance of 30% at 950 nm.
The use of standard polyesters combined with biaxial orientation of the films described makes them unsuitable for thermoforming processes.
The films described are moreover unsuitable for outdoor applications because prolonged insolation impairs their mechanical and optical properties. Because the IR-absorbing dyes used are sensitive organic molecules which are decomposed by insolation, the IR-filter action of the films described rapidly reduces in outdoor applications.